1. Field of the Invention
This invention relates to underwater sonar transducers and is directed more particularly to an acoustic transducer having stacks of piezoelectric ceramic transduction plates therein and having means for placing substantial compressive prestress on the ceramic plates and for relaxing the compressive stress to permit removal and replacement of one or more stacks of ceramic plates.
2. Description of the Prior Art
An underwater sonar transducer of the type described herein consists, in general, of a shell of some specified length which is hollow and of a generally elliptic cross section. The shell typically houses one or more stacks of piezoelectric ceramic plates and is designed to place a substantial compressive prestress on the ceramic plates. The plates of each stack are typically bound together, as by adhesive, though not in all cases. When an alternating voltage is placed on the piezo-electric plates, they expand and contract in such manner as to drive the narrow ends of the elliptical shell. This is transformed into large motions at the broad surfaces of the ellipse which are the major acoustic energy radiating surfaces.
Transducers of this general type are known and the elliptical shell may be of metal, formed to the desired dimensions with the desired internal space for carrying the stack of ceramic piezoelectric members, or it may be of a material such as glass fiber in an epoxy matrix. In either case, the one piece shell must be compressed significantly or flattened to increase the length of its hollow interior chamber so that the stack of ceramic plates can be inserted, after which the compressive force is removed, and the shell tends to return to its original shape, thus applying a static compressive pre-stress on the stack. It will be apparent that it is difficult to design and build a shell and a transducer stack wherein the dimensions of each are such as to provide just the right amount of prestress on the ceramic stack.
Further, once the transducer is assembled and the ceramic transduction plates are under prestress, it is difficult to extract from the shell a stack having one or more broken plates and to insert a new stack of transduction plates in place of the stack having the broken plate or plates.
It is therefore deemed beneficial to have means for exerting prestress on the transduction plates after the plates have been assembled in stacks and placed in a housing. It is further deemed beneficial that such means for exerting prestress are also adapted to relax compression of the plates, such that a stack may be removed from a housing and a new stack inserted into the housing.